Blender crashes when manipulating a large number of vertices with proportional editing enabled
It would probably be better to use a lattice modifier but if you wish to deselect vertices the following method seems to work for this example. Select the area you wish to manipulate then go to the 3D viewport header choose: Select > Random, go to the bottom of the tool shelf: enter Percent> 99% (for example) and Action> Deselect.You may need to add a few vertices to fill in any gaps. You can now manipulate using the proportional editing tool set to a largeish fall off without unwelcome lag or crashes.